The stability of ground ice in the equatorial region of Mars

Abstract

Consideration of the partial pressure of H2O in the Martian atmosphere and the range of mean annual temperatures at the Martian surface suggests that the occurrence of ground ice in equilibrium with the atmosphere is restricted to latitudes poleward of ±40°. However, there is a growing body of morphologic evidence that indicates that substantial quantities of ground ice may have been present in the equatorial regolith throughout Martian geologic time. The accepted explanation for this apparent contradiction has been that the H2O found near the equator is a relic, emplaced very early in Martian geologic history (>3.5 b.y. ago) and under substantially different climatic conditions. It is generally believed that this fossil ground ice layer has been preserved to the present day by the diffusion‐limiting properties of a relatively shallow layer ( ≲10 m) of fine‐grained regolith. To evaluate this hypothesis, the lifetime of a 200‐m layer of ground ice, buried below 100 m of ice‐free regolith, has been examined for the latitudes between ±30° on Mars. Twelve model pore size distributions, representative of silt‐ and clay‐type soils found on Earth, were selected to simulate the pore structure of the Martian regolith. The parallel pore model of gaseous diffusion was then used to calculate the flux of escaping H2O molecules from the buried ground ice layer. The potential effects of such factors as depth of burial, the Martian geothermal gradient, regolith porosity, adsorption, surface diffusion, and climatic change were also considered. Based on our analysis, the most important factors affecting the stability of equatorial ground ice appear to be (1) soil structure, (2) the magnitude of the geothermal gradient, and (3) the climatic desorption of CO2 from the regolith. We conclude that while the conditions necessary for the long‐term survival of ground ice may exist in isolated regions within the equatorial regolith, on the global scale it appears unlikely that a fossil ground ice layer has been preserved throughout Martian geologic history. If massive ground ice does presently exist in the equatorial regolith, then its presence may imply the existence of some continuous or periodic process of replenishment.